Making Sense of the Tap Rules

Last month we discussedSection 240-21 tap rules, and the effectiveness of the overcurrent protection for the tapped conductors. Sec. 240-21(b)(4) Taps Over 25 ft. Long is similar to the 25-foot tap in (b)(2), with these added restrictions:

• The installation is in a manufacturing building with walls over 35 feet high;

• Conditions of maintenance and supervision ensure that only qualified persons will service the system;

• The tap conductors are not over 25 feet long horizontally and not over 100 feet in total length;

• The tap conductors are continuous, containing no splices;

• The tap conductors are No. 6 copper or No. 4 aluminum or larger;

• The tap conductors do not pass through any floor, wall, or ceiling; and

• The tap is made at a location not less than 30 feet from the floor.

The condition regarding service personnel qualifications is impossible for the authority having jurisdiction to enforce. The situation at the time of the initial inspection may change. The proposal that was accepted for the 1981 NEC failed to pass for the 1978 edition, and was held over. The only supporting comment was that this arrangement had been used successfully. The proposal was made for the sole purpose of legitimizing those installations that violated the Code. There was no technical support for this change. [1981 Preprint, Pg. 64, Proposal 12].

If this 100-foot tap is safe, then why is (b)(2) limited to 25 feet? The longer the tap, the more likely it is to have a ground fault or short circuit. Are the vertical portions of the tap less likely to develop a fault than the horizontal portions? I do not understand or support this tap rule.

240-21(b)(5) Outside Taps of Unlimited Length. These tap conductors shall be protected from physical damage: if overhead, by elevation and other rules in Article 225; if underground, by Section 300-5 requirements. The tap must terminate in a disconnecting means located either outside a building or structure, or inside nearest the point of entrance of the conductors, and in a single circuit breaker or set of fuses that limit the load to the ampacity of the tap conductors.

The supporting comment for the proposal was that many such installations exist as services, with a good safety record, and therefore should be even safer with overcurrent protection ahead of the tap. This argument seems logical, but the lack of a minimum size for the tap conductors seems to be a flaw. The unlimited length and possibly small size relative to the overcurrent protection ahead of the tap could result in serious trouble in case of a fault if the overcurrent protection did not open quickly. However, because the tap is outdoors, the hazard to buildings is eliminated. The single overcurrent device at the load end definitely protects the tap against overload.

Sec. 240-21(c) Transformer Secondary Conductors. Section (c)(1) Protection by Primary Overcurrent Device is a repeat of Sec. 240-3(f), which states that transformer secondary conductors are not protected by the primary overcurrent device except for two-wire to two-wire single-phase and three-wire delta to delta three-phase.

240-21 (c)(2) Transformer Secondary Conductors Not Over 10 ft. Long. In the 96 NEC, this 10-foot tap was either from a feeder or from a transformer secondary. Those have been separated in the ’99 NEC, and these requirements are identical to those of 240-21(b)(1) with the exception that (b)(1)(d) has been omitted, which required the overcurrent ahead of the 10-foot tap to be not more than 10 times the ampacity of the tap conductors.

240-21(c)(3) Secondary Conductors Not Over 25 ft. Long. In the ’96 NEC, the 25-foot tap in Sec. 240-21(b)(2) did not apply to transformer secondaries, but this new Section broadens that coverage, but limits it to “industrial installations only.” There is another significant difference: the load end overcurrent is not limited to a single device, but can be up to six, complying with the requirement for transformer overcurrent protection in Sec. 450-3, where the secondary overcurrent device is permitted to be up to six circuit breakers or six sets of fuses, the total ratings not to exceed the required rating of a single device. This arrangement lends itself to future violations by making it easy to add more load.

240-21(c)(4) Outside Secondary Conductors. In the ’96 NEC, this tap rule applied to taps from a feeder or from a transformer secondary. Those have been separated, the requirements here being identical to those of Sec. 240-21(b)(5). One puzzle: Why does this transformer secondary tap rule not permit up to six circuit breakers or six sets of fuses at the load end?

240-21(c)(5) Secondary Conductors From a Feeder Tapped Transformer. This confirms that Sec. 240-21(b)(3) is still operative, for that primary is tapped from a feeder, while the secondary is tapped from the transformer, so it does not neatly fit into either (b) or (c) without this explanation.
Next time, we will finish our review of Sec. 240-21 by looking at 240-21(d), (e), (f), and (g), which are references to other articles where there are taps rules for service conductors, busways, motors and generators.

We will also look at Part H of Article 240, Supervised Industrial Installations, where there are different tap rules.

SCHWAN is an electrical code consultant in Hayward, Calif. He can be reached at bevschwan@aol.com.